There are growing concerns about the rapid spread of exotic plants into riparian zones, yet little information is currently available on their influence on stream ecosystems. This study assessed the impact of riparian invasion by Japanese knotweed (Fallopia japonica), an aggressive invader, on leaf litter breakdown and its associated biota (aquatic hyphomycete fungi and benthic invertebrates) in heterotrophic, low-order streams in The Pennines (England) and the Pyrenees (France). Our results suggest that leaf consumers (aquatic hyphomycetes and invertebrate shredders) can readily use knotweed leaf litter even in stream sites where it was not previously present. However, aquatic hyphomycete and invertebrate assemblages differed between stream sites with and without knotweed. Leaf litter breakdown rate and relative abundance of large invertebrate shredders (mainly Trichoptera) were enhanced in the Pyrenean invaded site, whose channel contained a high proportion of knotweed leaf litter, whereas no such effects were observed in The Pennines, possibly because of the less extensive knotweed invasion. Alteration of riparian vegetation by plant invaders could therefore increasingly influence instream community and ecological functions as the severity of invasion rises.